The invention relates to an improved process for the production of pure styrene from a pyrolysis gasoline fraction. Processes used to recover pure styrene of polymerisation grade from pyrolysis benzene fractions by extractive distillation are known from the documents listed hereafter: U.S. Pat. No. 3,684,665 by Toray and publications by Sato, Hydrocarbon Processing, May 1973, pages 141 ff, Morimoto et al., Bulletin of the Japan Petroleum Institute, Vol. 16, No. 1, May 1974, pages 38 ff, Gentry et al., Hydrocarbon Processing, June 1997, pages 73 ff., Emmrich et al., Int. J. of Hydrocarbon Engineering, Vol. 3, No. 9, October 1998, pages 62 ff, and Gentry et al., Hydrocarbon Processing, June 2004, pages 62 ff.
The state-of-the-art standard method encompasses the following processing steps:    (1) Separation of a C7− fraction as top fraction in a first distillation column and recovery of a C8+ fraction as bottom fraction;    (2) Separation of a C9+ fraction in the bottom of a second distillation column and recovery of a C8 fraction at the column top;    (3) Selective hydrogenation of phenylacetylene in the C8 fraction;    (4) Feeding the selectively hydrogenated C8 fraction to an extractive distillation column and performance of the distillation to recover a styrene solvent fraction in the bottom and a fraction poor in styrene at the column top;    (5) Post-treatment of the styrene fraction to remove chromophores;    (6) Final treatment of the styrene fraction that has undergone post-treatment to remove by-products formed during post-treatment.
Step (3) of the selective hydrogenation of phenylacetylene is required to remove this component from the input stream to the extractive distillation. Phenylacetylene (=phenylethine) like styrene is an unsaturated compound. Its boiling point is approx. 142.4° C., i.e. very close to the boiling point of styrene (145.8° C.). Hence, a simple distillative separation of styrene is impossible. On account of its polarity, phenylacetylene cannot be removed by extractive distillation from styrene even in the presence of a selectively acting solvent, as the affinity of phenylacetylene to the selectively acting solvent is stronger than that of styrene to the said solvent. In this case, phenylacetylene together with styrene would be obtained as extract from such an extractive distillation.
The specification of pure styrene prescribes that the phenylacetylene content of pure styrene must not exceed a certain limit value. It is therefore necessary to remove the phenylacetylene from the styrene-bearing stream upstream of the extractive distillation unit. One method of such a removal is selective hydrogenation (3). In this process step, phenylacetylene is converted to styrene and/or ethyl benzene with the aid of hydrogen. A typical embodiment of this process step is to hydrogenate the phenylacetylene during the liquid phase at elevated pressure and under moderate temperature conditions. A catalyst is also required for this step. The specialist skilled in the art, as a rule, makes use of a noble metal catalyst (e.g. Pt or Pd) which is applied to a carrier material (such as Al2O3, SiO2 or TiO2).
With regard to the hydrogenation conditions, the pressure, temperature, retention time and catalyst quantity are optimised in such a manner that the phenylacetylene content of the reactor effluent reaches so low a level that during further styrene recovery the remaining phenylacetylene content of the styrene does not exceed the limit value and that the styrene loss due to overhydrogenation is minimised.
The following post-treatment methods in accordance with step (5) are known: U.S. Pat. No. 3,763,015 describes the treatment with a 60% nitric acid HNO3 and DE 198 53 916 the treatment with maleic acid anhydride (MSA). The disadvantage of the two said methods, however, is that further chemicals are in fact required for post-treatment, the high equipment expenditure for the necessary reaction vessels and reprocessing steps as well as the higher styrene loss due to the treatment with HNO3 or MSA, respectively, mainly caused by styrene polymerisation.
The said post-treatment is required because the styrene product obtained by step 0 is in fact in line with ASTM D-2827 requirements for the impurities contained, but it is not in accordance with the requirement for the colour of the product. This is set forth particularly in publications by Sato 1973, Morimoto 1974 and Emmrich et al., 1998. The above-mentioned ASTM code requires that the colour be ≦10 mg Pt/l (according to ASTM D-1209). The styrene product obtained by extractive distillation, however, has a light up to dark yellowish coloration, depending on the origin of the pyrolysis gasoline. This coloration is primarily due to the presence of 1,3-cyclopentadiene derivatives or other polyunsaturated, conjugated systems.
These are typical substances of the said group:    5-methylene-1,3-cyclopentadiene (fulvene)
    5-ethylidene-1,3-cyclopentadiene (6-methylfulvene),
    5-(1-methylethylidene)-1,3-cyclopentadiene (6,6-dimethylfulvene).
In general, the fulvenes constitute groups with the following parent substance:

These components and other non-cyclic, conjugated dienes and trienes are highly non-saturated components and absorb light in the visible area on account of their conjugated double-bond system. That is why these components have an intense coloration. Fulvenes, other cyclopentadiene derivates as well as further coloured, conjugated systems are hereinafter referred to as “chromophores”. A characteristic feature of the chromophores is that their molecules encompass chromophore groups. “Chromophore groups” are understood to mean such groups of atoms within a molecule that absorb light in the visible area. Fulvenes and other chromophores form during naphtha steam cracking. The boiling point of the fulvene parent substance is approx. 60° C. at atmospheric pressure and that of 6,6-dimethylfulvene is approx. 160° C.
On account of the boiling point of the chromophores, one may come to the conclusion that it would be preferable either to separate them with the C7− fraction in the primary distillation unit (step (1)) or to remove them with the raffinate fraction, poor in styrene, from the styrene in the extractive distillation unit (step (2)). Conjugated double-bond systems, such as those of the above-mentioned chromophores, are capable of reacting by way of a Diels-Alder reaction with dienophiles, in this case particularly with olefins. The compounds thus formed have higher boiling points than the components undergoing the reaction. The products originating from this reaction are hereinafter referred to as “Diels-Alder products”. The Diels-Alder reaction is of the reversible type so that the Diels-Alder products can decompose into the initial components, i.e. in this case the compounds with the conjugated double bonds and olefins.
The chromophores are “masked” as a result of the Diels-Alder reaction. Chromophores which initially were of the low-boiling type and should have been removable with the C7− fraction in the primary distillation, remain in the C8+ fraction, which again is distilled in a second distillation column. In this step, part of the Diels-Alder products are re-converted into chromophores and the corresponding dienophiles as a result of the high temperatures and the relatively prolonged retention time in the column bottom. Hence, the chromophores enter the C8 fraction and, in turn, part of the chromophores in the C8 fraction react with olefins to form Diels-Alder products. This phenomenon in particular occurs in the case of reflux vessels or buffer tanks for intermediate products, for example. These further Diels-Alder products again mask the chromophores. Thus it becomes possible that the chromophores masked as Diels-Alder products can penetrate the stripping column of the extractive distillation unit and here they are re-converted into their initial components, i.e. chromophores and olefins, because of the high temperatures especially prevailing in the stripping zone and the bottom of the column. In spite of their boiling point location, a certain part of the chromophores contained in the raw pyrolysis gasoline consequently enter the styrene fraction originating from the extractive distillation and cause a coloration of the product.